Duplicate column in CREATE TABLE statement

SQLite raises a parser error with the following DDL:
```sh
sqlite> CREATE TABLE test (
...> x TEXT,
...> x TEXT
...> );
Parse error: duplicate column name: x
```
But `sqlparser` doesn't:

```sh
% cargo run --example cli ddl.sql --sqlite
...
Round-trip:
'CREATE TABLE test (x TEXT, x TEXT)'
Parse results:
[
CreateTable {
...
```
Is this kind of checks considered out of scope ?
Thanks.

Reproduce the discrepancy with the SQLite DDL and the `cargo run --example cli ddl.sql --sqlite` command. Start by tracing how the CREATE TABLE statement is parsed, then determine whether duplicate column names should be rejected. Done means the parser's behavior is decided and covered consistently for this input.

Written by the indexing model from the issue text.